Patterns of shape-phase transition in the proton-neutron coupled systems are studied within the SD-pair shell model. The results show that some transitional patterns in the SD-pair shell model are similar to the U (5) − SU (3), U (5) − SO(6) transitions with signatures of the critical point symmetry of the interacting boson model. PACS numbers: 21.60.Cs * Electronic address: luoya@nankai.edu.cn Recently, based on the Generalized Wick Theory[1], a nucleon-pair shell model(NPSM) was proposed[2], in which nucleon pairs with various angular momenta are used as building blocks. Since modern computers fail for the calculation in the full shell model space for the medium weight and heavy nuclei, some truncation scheme need to be used. The tremendous success of the interacting boson model (IBM) [3], suggests that S and D pairs play a dominant role in the spectroscopy of low-lying nuclear modes [4, 5, 6]. Therefore, one normally truncates the full shell-model space to the collective S-D pair subspace in the NPSM. The latter is called the SD-pair shell model(SDPSM)[2, 7, 8]. A crucial point in the SDPSM is the validity of the S-D pair truncation. In Ref.[9], shell model foundations of the IBM was reviewed by Iachello, the results seem to indicate that the S-D pair truncation is a reasonable approximation to the full shell model space. Thisproblem was also studied in [10,11,12] with the conclusion that the S-D pair subspace works well in the vibrational region, but in the deformed region, the inclusion of G pairs is crucial.But Dr. Zhao's work [13,14] show that the essential properties within the full shell model space survive in the S-D pair subspace. What's more, if a pure quadrupole-quadrupole interaction and a reasonable collective S-D-pair were used, the rotational behavior can be